1. Field of the Invention
The present invention relates to a planographic printing plate having sensitivity in an infrared wavelength region and a method of producing the same. More particularly, the present invention relates to a negative planographic printing plate which can be obtained by so-called direct plate production in which plate production can be directly effected by using infrared laser based on digital signals from computers and the like.
2. Description of the Related Art
Recently, there are remarkable developments in lasers. Particularly, solid laser and semiconductor laser which emit an infrared ray having a wavelength from 760 nm to 1200 nm (hereinafter, sometimes referred to as xe2x80x9cinfrared laserxe2x80x9d), and show high output and have small size are easily available. These infrared lasers are very useful as a recording light source in directly producing a printing plate plates based on digital data from computers and the like. Therefore, there is, recently, increasing desire for an image recording material having high sensitivity, for such an infrared recording light source, namely, an image recording material whose solubility in a developer changes significantly due to irradiation with an infrared ray.
As such a negative image recording material which can be recorded by infrared laser, U.S. Pat. No. 5,340,699 describes a recording material composed of an infrared absorbing agent, an acid generator, resol resin and a novolak resin. However, such a negative image recording material requires, for image formation, a heating treatment after exposure with the laser. Therefore, a negative image recording material requiring no heating treatment after exposure has been desired.
Further, in a method using exposure of high power and intensity and utilizing a high output laser, an exposure region is irradiated intensively with a large amount of light energy for an exposure time of an instant. This light energy is converted efficiently into heat energy and thermal changes such as chemical changes, phase changes and, changes in form and structure are caused by this heat, and the changes are utilized for image recording. However, when the amount of an infrared absorbing agent added is increased for improvement of recording sensitivity in a recording layer of a conventional planographic printing plate, a laser exposure apparatus and a light source may become contaminated due to ablation (splashing) of the recording layer.
For achieving the two objects of improvement of image formation property and suppression of ablation, JP-A No. 11-192782 describes an image formation material having a structure containing two laminated recording layers having different functions respectively, however, this material is of positive type, and different from the present invention.
As a negative image formation material, WO 97/00777, for example, describes a negative image formation material having a photosensitive layer with a two-layer structure. However, there is a problem with this structure in that ablation tends to occur since the surface layer which is an exposure surface is photosensitive, and further, post-exposure is necessary for obtaining a strong image.
One object of the present invention is to provide a negative planographic printing plate which can manifest direct plate production by recording based on digital data from a computer and the like using solid laser and semiconductor laser emitting infrared rays, shows high sensitivity to infrared laser, suppresses ablation of a recording layer in recording, and has excellent image formation properties such as dot reproduction property and excellent printing endurance, and another object is to provide a preferable method of producing the same.
The present inventors have directed their attention to the property of a recording layer of a negative planographic printing plate which can realize direct plate production by irradiation with infrared ray and intensively studied this. As a result, they have found that the above-mentioned problems can be solved by causing the film hardness near the surface of a recording layer which is hardened by exposure with a laser to be higher than the film hardness of the entire body. Further, they found a plate production method for developing a planographic printing plate having such a recording layer using a developer having specific properties. These discoveries led to completion of the present invention.
Namely, the planographic printing plate of the present invention is a planographic printing plate comprising a substrate carrying thereon a recording layer which contains a polymerizable compound and an infrared absorbing agent, and causes a polymerization reaction by the action of light or heat to decrease solubility in an alkali developer, wherein the optical density of the recording layer is from 0.4 to 2.0, and the film hardness of the upper part of the recording layer after reduction in solubility in an alkali developer due to the action of light or heat is higher than the average film hardness of the recording layer.
In a preferable embodiment, the above-mentioned recording layer contains an infrared absorbing agent under conditions causing no ablation.
Further, in a certain embodiment, the method of producing a planographic printing plate of the present invention comprises exposing a planographic printing plate containing a substrate carrying thereon a recording layer which contains a polymerizable compound and an infrared absorbing agent, and causes a polymerization reaction by the action of light or heat to decrease solubility in an alkali developer, wherein the optical density of the recording layer is from 0.4 to 2.0, and the film hardness of the upper part of the recording layer after reduction in solubility in an alkali developer due to the action of light or heat is higher than the average film hardness of the recording layer, then, developing the plate by a developer showing lower permeability through the recording layer after hardening.
In the present invention, the expression xe2x80x9cby the action of light or heatxe2x80x9d also means xe2x80x9cby both of the action of light and the action of heatxe2x80x9d.
Though the action of the present invention is not explicit, it is speculated that at the exposed surface of a recording layer whose solubility in an alkali developer decreases by exposure at or near the exposed surface, sensitivity to infrared laser is excellent, film hardness due to a polymerization reaction is high, and in the deeper portions of a recording layer, film hardness becomes lower because of diffusion of heat to a substrate. However, when such a photosensitive layer is developed after hardening (polymerization reaction), portions around the surface of a recording layer manifest lower permeability to an alkali developer and function as a protective layer. Consequently, development stability becomes excellent and an image having excellent discrimination is formed, and simultaneously, stability long-term is also obtained.
Further, it is believed that, in production of a planographic printing plate having such a recording layer, by selecting a developer capable of optimum permeation through a recording layer based on electrostatic capacity described later and by effecting development with such a developer, the developer does not easily permeate portion in the vicinity of a substrate which have relatively lower hardness and both effects of increase in sensitivity and prevention of decrease in printing endurance can be satisfactorily achieved.